tools/android/loading: Fix cache pushing non deterministic bug on Android
tools/android/loading: Fix cache pushing non deterministic bug on Android

On Android, Sandwich create a cache of all resources by navigating to
the URLs and then give a slow death to chrome: switch it to background
so it can persist its cache on disk before killing it. However the next
sandwich run was getting non deterministic cache use, because chrome
was previously not killed enough.

This CL fix this non deterministic bug by using am force-stop command
line instead of kill -9 <pids>.

BUG=582080

Review URL: https://codereview.chromium.org/1902133002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388457}

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